A multiple input multiple output (MIMO) communication system comprises a transmitter station, which transmits signals via a wireless communications medium utilizing NTX>1 transmitting antennas, and a receiver station, which utilizes NRX≧1 receiving antennas to receive signals. Such a system may be referred to as an NTX×NRX MIMO system. By comparison, a communication system, which comprises a transmitter station, which transmits signals utilizing NTX=1, and a receiver station, which utilizes NRX=1 receiving antennas to receive signals may be referred to as a single input single output (SISO) communication system. The transmitting stations in a MIMO communication system may utilize the plurality of NTX transmitting antennas to concurrently transmit signals, which comprise data received from NSS≧1 distinct data streams, or spatial streams. In instances, when the transmitting station utilizes space time coding, the NSS spatial codes may be utilized to generate NSTS≧1 space time codes, which may then be utilized to generate signals, which may be concurrently transmitted by the NTX≧1.
In a MIMO communication system, a plurality of radio frequency (RF) channels may concurrently exist between the transmitting station and the receiving station. For example, each of the concurrently transmitting NTX transmitting antennas may establish an RF channel to each one of the NRX receiving antennas. Each of these channels may be referred to as a spatial channel. Consequently, in an NTX×NRX MIMO system supports a plurality of (NTX)×(NRX) spatial channels. In MIMO communication systems, which utilize orthogonal frequency division multiplexing (OFDM), a plurality of fc distinct frequency carriers, or tones, may be utilized within the RF channel bandwidth. Each of the fc tones may be concurrently transmitted via each RF spatial channel. For example in IEEE 802.11n WLAN systems, which utilize OFDM, fc=56 for 20 MHz bandwidth or fc=112 for 40 MHz bandwidth. Within a given RF spatial channel, each of the tones may be referred to as an RF subchannel. Thus, an NTX×NRX MIMO system supports a plurality of (fc)×(NTX)×(NRX) RF subchannels.
MIMO communication systems may be utilized to increase communication throughput (the rate at which data is communicated as measured, for example, in bits per second (BPS)) and/or to increase communication reliability (as measured, for example, by bit error rate (BER)) relative to SISO communication systems. In instances when NTX=NSS=NRX, the MIMO communication system may maximize the aggregate data transfer rate between the transmitting station and the receiving station. In instances when NTX>NSS and NRX≧NSS, the MIMO communication system may increase communication reliability through the utilization of diversity transmission. In a diversity transmission system, data from a given spatial steam may be concurrently transmitted via a plurality of transmitting antennas. In instances when NSS=1, the MIMO communication system may utilize diversity maximization.
MIMO communication systems may also achieve diversity transmission through the use of STC techniques such as space time block coding (STBC) or space frequency block coding (SFBC). In MIMO communication systems, which utilize STC, the NSS spatial streams, each of which may comprise a sequence of data symbols, or codewords, are transformed into NSTS space time streams. The NSTS space time streams may enable the NTX transmitting antennas to transmit signals comprising an aggregate of L codewords over a time duration of TSTC time units, where TSTC refers to a time duration for STC processing. In an STBC MIMO communication system, a given codeword, or a transformed version of the codeword (for example, a complex conjugate version), may be transmitted multiple times within the TSTC time duration with each transmission occurring from a different one of the NTX transmitting antennas.
In order to increase throughput for either data rate maximization or diversity transmission, the transmitter station may attempt to focus the transmitting energy in the direction of a receiver station. The focusing of transmit energy may increase the signal to noise ratio (SNR) of the signals received at the receiver station. By increasing the SNR ratio, the transmitter station may increase the data-carrying capacity of the RF channels, thereby increasing the potential throughput. The transmitter station may focus the transmitting energy by a technique referred to as beamforming. The transmitting station may generate beamformed signals by assessing the characteristics of the propagation path of the RF channels through the wireless communication medium between the transmitting station and the receiving station. The transmitting station may accomplish this assessment through the computation of channel estimates.
In a closed loop MIMO communication system, the transmitter station may compute channel estimates based on feedback information received from the receiver station. The receiver station may compute channel estimates based on signals received from the transmitter station. The computed channel estimates may be referred to as channel state information (CSI). The CSI may be represented as a channel estimate matrix H. The receiver station may communicate the channel estimate matrix H to the transmitter station within the feedback information. The transmitter station may utilize the fed back channel estimate matrix to generate beamformed signals, which are transmitted to the receiver station.
One limitation in closed loop MIMO communication systems is the quantity of feedback information, which is transmitted from the receiver station to the transmitter station represents overhead. The quantity of overhead reduces the available channel capacity for the communication of data. Thus, the overhead may reduce RF channel throughput. The channel estimate matrix H may comprise a plurality of (fc)×(NTX)×(NRX) matrix elements, one matrix, hij, element for each RF spatial channel between the ith transmitting antenna at the transmitter station to the jth receiving antenna at the receiver station. In addition, in a MIMO communication system that utilizes OFDM, for each hij matrix element, there are fc matrix elements, one for each OFDM tone. In instances when each matrix element is represented by an nh bit binary word, the quantity of feedback information due to the channel estimate matrix H is (nh)×(fc)×(NTX)×(NRX) bits.
Closed loop MIMO communication systems may enable the transmitter station to more accurately characterize the propagation path from the transmitter station to the receiver station in comparison to open loop MIMO communications when the transmitter station and the receiver station are stationary. Another limitation in closed loop MIMO communication systems is the tendency for the CSI feedback data to become stale when the transmitter station and/or receiver station are mobile or when surrounding environment is dynamic. In the case of mobility, the motion may cause changes in the characteristics of the propagation path between the transmitter station and the receiver station. In such cases, the transmitter station may end up utilizing CSI, which was previously received from the receiver station, which may no longer provide an accurate representation of the RF channel. Such CSI may be referred to as being stale. The use of stale CSI by the transmitter station for the purpose of generating beamformed signals may result in reduced throughput. However, attempting to compensate for the tendency of CSI staleness by increasing the frequency with which the receiver station transmits updated CSI to the transmitter station increases overhead and reduces available throughput.
In an open loop MIMO communication system, the transmitter station may transmit signals without utilizing feedback information received from the receiving station. Transmitter stations in open loop MIMO communication systems typically utilize STC.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.